The present invention relates to a process for the production of hydrocarbons with a high octane number, obtained by the selective dimerization reaction, in a tubular reactor, of the isobutene contained in hydrocarbon cuts, characterized by a low isobutene content and a high linear olefin/isobutene ratio ( greater than 3), which favours the production of higher selectivities on the part of the catalyst. The mixture obtained can then be hydrogenated with the conventional methods to obtain a product with further improved octane characteristics.
At present, refineries throughout the whole world are producing xe2x80x9cEnvironmental Low Impact Gasolinesxe2x80x9d (characterized by a reduced content of aromatics, olefins, sulfur and a lower volatility), obviously with the aim of minimizing the effect of their production on the functioning of the refinery itself.
MTBE and alkylated products are the most suitable compounds for satisfying future refinery demands; however, at the moment, the use of MTBE is extremely risky and alkylated products are not easily available.
The ban of MTBE from gasolines in California and the continual attacks to which it is subjected, owing to its presumed toxicity, have in fact jeopardized its use (and also that of other alkyl ethers) in future reformulated gasolines. The removal of this ether will create considerable problems in refineries as MTBE, in addition to its high-octane function, also has a diluting action of the products which are most harmful for the environment (sulfur, aromatics, benzene, etc.,). The alkylated product is undoubtedly the ideal compound for reformulated gasolines, as it satisfies all the requisites provided for by future environmental regulations owing to the combination of a high octane number with a low volatility and a complete absence of olefins and aromatics.
Another positive aspect of alkylation is its capacity to activate isoparaffinic hydrocarbons, such as isobutane, for example, which binds itself, by reaction in liquid phase catalyzed by strong acids, with olefins (propylene, butenes, pentenes and relative mixtures) creating saturated C7-C9 hydrocarbons with a high octane number.
Quantities of alkylated product, however, greater than those at present available, would require the construction of large alkylation units as at present, owing to its scarcity, it is not a commodity which is widely available on the market but is a gasoline component used exclusively in refineries where it is produced.
This represents a serious limitation in the use of alkylated products on a wide scale, as the construction of new units is limited by the incompatibility of the catalysts used in traditional processes (hydrofluoric acid and sulfuric acid) with the new environmental regulations; the process with hydrofluoric acid owing to the toxicity of this acid, especially in populated areas, and the process with sulfuric acid owing to the large production of acid mud as well as the considerable corrosive capacity of the catalyst.
Alternative processes are being developed with solid acid catalysts but their commercial applicability has still to be demonstrated.
To solve these problems, it will therefore be necessary to resort more and more to the use of purely hydrocarbon products, such as those obtained by the selective dimerization of C3 and C4 olefins, which owing to their octane characteristics (high Research Octane Number (RON) and Motor Octane Number (MON)) and also their boiling point (limited volatility but low end point) are included in the range of compositions which are extremely interesting for obtaining gasolines which are more compatible with present-day environmental demands.
The oligomerization process (often incorrectly called polymerization) was widely used in refining in the xe2x80x9830sxe2x80x99 and xe2x80x9840sxe2x80x99 to convert low-boiling C3-C4 olefins into the so-called xe2x80x9cpolymerxe2x80x9d gasoline. Typical olefins which are oligomerized are mainly propylene, which gives dimers (C6) or slightly higher oligomers depending on the process used, and isobutene which mainly gives dimers (C8) but always accompanied by large quantities of higher oligomers (C12+).
This process leads to the production of a gasoline with a high octane number (RON about 97) but with a high sensitivity due to the purely olefinic characteristic of the product (for more specific details on the process see: J. H. Gary, G. E. Handwerk, xe2x80x9cPetroleum Refining: Technology and Economicsxe2x80x9d, 3rd Ed., M. Dekker, New York, (1994), 250). The olefinicity of the product obviously limits the process as the hydrogenation of these mixtures always causes a considerable reduction in the octane characteristics of the product, which thus loses its appeal.
If we limit our attention to the oligomerization of isobutene, it is known that this reaction can be carried out batchwise, in semi-continuous and in continuous, both in gas-solid phase and in liquid phase, generally at temperatures ranging from 50 to 300xc2x0 C. and a atmospheric pressure or at such pressures as to keep the reagents in liquid phase, if considered necessary.
The dimerization of isobutene is generally carried out with acid catalysts such as phosphoric acid, generally supported on a solid (for example kieselguhr), cationic exchange acid resins, liquid acids such as H2SO4, sulfonic acid derivatives, silico-aluminas, mixed oxides, zeolites, fluorinated or chlorinated aluminas, etc.
The main problem of dimerization, which has hindered its industrial development, is the difficulty in controlling the reaction rate; the considerable activity of all these catalytic species, together with the difficulty in controlling the temperature in the reactor, in fact, makes it extremely problematical to limit the addition reactions of isobutene to the ever lengthening chains and, consequently to produce a high quality product characterized by a high selectivity to dimers.
In the dimerization reaction, there is, in fact, the formation of excessive percentages of heavy oligomers such as trimers (selectivity of 15-60%) and tetramers (selectivity of 2-10%) of isobutene. Tetramers are completely excluded from the gasoline fraction as they are too high-boiling and therefore represent a net loss in yield to gasoline; as far as trimers are concerned (or their hydrogentated derivatives), it is preferable to greatly reduce their concentration, as their boiling point (170-180xc2x0 C.) is on the limit of future specifications on the final point of reformulated gasolines.
From what is specified above, it is evident that there is great interest in obtaining a new dimerization process of isobutene which allows the synthesis of a higher quality product, by reaching greater selectivities.
It has now been surprisingly found that high selectivities can be obtained by using hydrocarbon charges which are particularly rich in linear olefins and by carrying out the dimerization reaction of isobutene in a tubular reactor capable of removing the heat as it is generated. Operating as such, it is possible to obtain the production of an oligomer fraction particularly rich in dimers ( greater than 80% weight).